During reconstruction, republicans dominated new state governments because of

The Reconstruction Act of 1867

Explanation:

During the Reconstruction, the African Americans were for the first time offered a series of right they did not have before, one of those rights was the right to vote and participate in politics.

The Reconstruction Act of 1867 started the period known as the Radical Reconstruction. Republican governments came to power in the Southern States and offered African Americans a share of political power, they also established public schools, civil rights laws and economic development.

By offering blacks a general share of political power, republicans dominated new state governments under the terms of the reconstruction Act of 1862.